The explosive growth in mobile electronic device usage such as smartphones and tablets creates an increasing need in the art for compact and efficient switching power converters so that users may recharge these devices. Flyback switching power converters are typically provided with mobile devices as their transformers provides safe isolation from AC household current and high efficiency. In a flyback switching power converter, a power switch cycles on and off to transfer power to a load through a transformer. A flyback switching power converter will thus produce switching noise that depends upon the frequency of the power switch cycling.
The resulting switching noise may interfere with the touchscreen operation in the powered smartphone or tablet. In that regard, a touchscreen may be driven with a sensing signal having a certain frequency such as 300 KHz. If the flyback switching noise is within the same frequency band used for the touchscreen sensing signal, the touchscreen operation may have interference from the switching noise. Modern high-sensitivity touchscreens are particularly susceptible to such interference. For example, touchscreens have been developed such that users may interact with the touchscreens even while wearing gloves. This increased sensitivity for touchscreens heightens their sensitivity to flyback switching power converter switching noise. Given this sensitivity, it is conventional for a touchscreen controller to monitor the noise across potential frequency bands for the sensing signal so that the sensing signal frequency may be chosen accordingly. For example, a flyback switching power converter may operate at a certain switching frequency such as 80 KHz. The resulting switching noise will then be centered not only around 80 KHz but also at harmonics of the power switch cycling frequency such as 160 KHz, 240 KHz, 320 KHz, and so on. The touchscreen controller may then choose 300 KHz for its signal sensing frequency as the noise will be relatively light in between the 240 KHz and 320 KHz harmonics.
If the power switch cycling were then fixed such as at 80 KHz, a touchscreen controller could readily avoid switching noise. But a flyback switching power converter must also meet certain load efficiency standards. To satisfy the required efficiency at a low load such as at 10% of maximum power, it is conventional for a flyback switching power converter controller to transition out of a fixed frequency pulse-width-modulation (PWM) mode used for higher load states to a pulse frequency modulation mode (PFM) in which the pulse width is fixed but the pulse frequency is reduced as the load drops. For example, the switching frequency of the PFM mode may vary from some maximum frequency such as 80 KHz to a much lower frequency such as 100 Hz. The resulting wideband operation makes it difficult for the touchscreen controller to find a noise-free band for driving its sensing signal.
Accordingly, there is a need in the art for improved switching power converters that maintain efficiency while reducing switching noise.